High-angular resolution observations of the Pistol Star

High-angular resolution observations of the Pistol Star
Christophe Martayan, Ronny Blomme, Jean-Baptiste Le Bouquin, Antoine
Merand, Guillaume Montagnier, Fernando Selman, Julien Girard, Andrew
Fox, Dietrich Baade, Yves Fremat, et al.
To cite this version:
Christophe Martayan, Ronny Blomme, Jean-Baptiste Le Bouquin, Antoine Merand, Guillaume
Montagnier, et al.. High-angular resolution observations of the Pistol Star. poster at IAUS272.
2010. <hal-00526826>
HAL Id: hal-00526826
https://hal.archives-ouvertes.fr/hal-00526826
Submitted on 15 Oct 2010
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Active OB stars: structure, evolution, mass loss and critical limits
c 2011 International Astronomical Union
Proceedings IAU Symposium No. 272, 2011
C. Neiner, G. Wade, G. Meynet & G. Peters, eds.
DOI: 00.0000/X000000000000000X
High-angular resolution observations of the
Pistol Star
C. Martayan1,2 , R. Blomme3 , J.-B. Le Bouquin4 , A. Merand1,
G. Montagnier1 , F. Selman1 , J. Girard1 , A. Fox1 , D. Baade5 ,
Y. Fremat3 , A. Lobel3 , F. Martins6 , F. Patru1 , Th. Rivinius1 ,
H. Sana7 , S. Štefl1 , J. Zorec8 and T. Semaan2
1
ESO Chile; 2 GEPI-Observatoire de Meudon, France; 3 Royal Observatory of Belgium,
Brussels, Belgium; 4 Laboratoire d’Astrophysique de Grenoble, France;
5
ESO Germany; 6 GRAAL, Montpellier, France; 7 Amsterdam University, The Netherlands;
8
Institut d’Astrophysique de Paris, France
Abstract. First results of near-IR adaptive optics (AO)-assisted imaging, interferometry, and
spectroscopy of this Luminous Blue Variable (LBV) are presented. They suggest that the Pistol
Star is at least double. If the association is physical, it would reinforce questions concerning the
importance of multiplicity for the formation and evolution of extremely massive stars.†
Keywords. stars: binaries, stars: circumstellar matter, stars: early-type, stars: mass loss
1. Introducing the Pistol Star
At the time of its formation ∼2 million years ago, the Pistol Star may have been one
of the most massive stars in the Milky Way (Figer et al. 1998). Like most other LBV’s,
it is surrounded by nebulosities, which, therefore, may actually have been ejected by the
Pistol Star (cf., e.g., η Car). The shape of the main gas cloud has earned the star its
nick name. Owing to its own ejecta but mostly due to its far-away location (∼ 8 kpc) in
the region of the Galactic Center, observations of the Pistol Star suffer from very high
extinction (20-30 magnitudes in the optical).
Among massive stars, the fraction of binaries may approach 100% at the time of formation (cf. Mason et al. 2009, Sana et al. 2009 ). In fact, binarity may be one means
to enable the formation of massive stars, which with single-star-plus-disk symmetry encounters challenges already at the 10-M⊙ level while observations of LBVs imply original
masses that are larger by more than an order of magnitude. One of the most prominent
massive stars reported to be a binary is η Car (Damineli, Conti, & Lopez 1997).
2. Observations
In order to more closely examine the nature of the Pistol Star, near-IR observations
were undertaken with the VLT AO camera NACO and the VLTI spectrograph AMBER.
The angular resolutions of NACO, ranging from natural seeing to near the defraction
limit at ∼15 mas, and AMBER, 20-3 mas, are perfectly complementary to one other. At
a distance of 8 kpc, this permits spatial scales to be studied down to ∼25 AU. The new
VLT spectrograph X-Shooter provided spectra with R ∼ 5000 − 11000 over the range
0.3-2.5 µm. They show a vast number of nebular lines; the stellar continuum begins to
emerge at ∼ 800 nm. At the time of the Symposium, some additional observations were
pending, also of other massive stars.
† based on ESO runs 085.D-0182(A), 085.D-0625(A, C)
1
2
C. Martayan, R. Blomme, J.-B. Le Bouquin et al.
Figure 1. NACO image (2.3x2.2 arcsec) of the Pistol Star. Note the appearance of several
nearby point sources, while the Pistol Star itself is surrounded by extended nebulosity, measuring
roughly 700 AU across. The structure in the nebulosity is not due to discontinuities in the color
coding but believed to be real, possibly evidencing multiple shells.
3. First preliminary results
The NACO images resolve most of the stars within 30 arcsec of the Pistol Star into
several point sources. Fig. 1 depicts the immediate vicinity of the Pistol Star itself as
observed with NACO in K band. Apart from several point sources, it reveals an extended
nebulosity. At a distance of 8 kpc, its diameter would correspond to more than 6000 AU.
Figer et al. 1998 find gas velocities with a range of ∼50-100 km/s but over a larger area.
Combining these numbers leads to a crude estimate of the expansion age of 290 years.
If the inference from Fig. 1 of multiple shells is correct, they would differ in age by
∼30 years.
A quick preliminary inspection of the AMBER interferometry suggests the presence
of a further point source at a separation of about 10 mas or ∼ 80 AU. For comparison:
If the 5.5-year period in η Car is orbital, its companion would be at one-fifth or less of
this distance. In the NACO images, the inner nebulosity looks less structured than in the
case of η Car. An effort will be made to continue the observations in order to establish
whether the Pistol Star has a physical companion with effects on formation and evolution
of the system.
References
Damineli, A., Conti, P.S. & Lopez, D.F. 1997, New Astron., 2, 107
Figer, D.F., Najarro, F., Morris, M., McLean, I.S., Geballe, Th.R., Ghez, A.M. & Langer, N.
1998, ApJ, 506, 384
Mason, B.D., Hartkopf, W.I., Gies, D.R., Henry, T.J. & Helsel, J.W 2009, AJ, 137, 3358
Sana, H., Gosset, E. & Evans, C. J. 2009, MNRAS, 400, 1479